I disagree, for charging (balance charging) like what i was talking about i feel series is safest option, to get well balanced match cells to use them in series as balance charging goes on voltage not mAh, You can not get over discharge thermal run away from series balance charging, but this is why i feel hobby chargers and balance charging with a decent charger & DMM is important for multi cell lights, but with out a hobby charger you will never know if the cells match or not and most don't know if a cell is any good or not.
I also feel 0.5 volt difference between Lion cells is enough to get spikes from 0 to 5 amps depend on the cell and how many cells, one cell is trying to discharge and the next cell is trying to be charged every time there is a voltage difference with parallel charging or connecting batteries in parallel, it is not good to discharge or charge batteries that high (1.5 to 2c for 18650's) even if it is short burst, it will affect the life of the battery the internal resistance, the end/resting voltage voltage and how much amps it can give over time.
With other li-xx type batteries say li-po,s this number is much much higher say in the 100's, see i like lower charge rates but with as many cells as i can in series, over the years i have proven to myself it reduces the wear on the cells keeps the internal resistance lower for longer and the cells are balanced the best you can with regarding the internal resistance age and abuse and so on.
The quick short spikes from unmatched cell voltage with parallel charging will increase the internal resistance quicker then normal use if your not on to it due to the cells trying to charge and discharge them self,s very quickly, when they are connected in parallel, say if some one is new to hobby chargers where you have to think and match cells or voltages your self with parallel i don't thing that is a good idea.
I do understand where your coming from and with parallel charging if the cells are matched in voltage you can set the hobby charger to cut off for the mAh not just the voltage so you can have even cells that way, but this is tricky as cells age different and if they have diffrent internal resistance, and people using dedicated simple chargers no DMM will never know if the cells are slightly out in voltage or mAh and internal resistance, and the spikes do occur not just with charging but every time you put them in a parallel srt up like a light even if they come of the charger with a green light.
This talk is like whats best ford or Holden and so on, i know what works for me what is easier and what is proven to me by me and what Ive been using for RC for 20 years, all RC chargers (we call them hobby chargers on here) are made for series charging, all battery packs are charged and discharged in series or are mixed in a few examples (like laptops), but with more volatile type cells like li-xx this is where balancing came in, but each person will have there own point of view and will like to do it there way, but i will recommend what i thinks best for charging and whats the easiest way for the masses with little to no experience.
You will never get thermal run away from balance charging, infact when using batteries in series like in a multi cell light or an RC car its pretty dam hard to get thermal run away from over discharging, for one most have low voltage warning well my cheap trustfires do and so do my RC,s, most cells have a semi working built in protection, some thing major has to fail electronically, dead shorts mixed with a failed PCB or battery has to drop out all together, even then its more likely to be user error.
I have used a cheap set of batteries from byincoins in a multi cell light with a current draw of 2 amps over two cells one cell died it was reading 0.9 volt the other 4.1 volt and i still have all my fingers but the torch did dim witch gave the warning out not to keep pushing it, i have even dead shorted a good trustfire flame from manafont with no PCB for over 5 Min's and it did not explode and i have used many batteries in my 5 odd multi cell lights with diffrent internal resistance mAh and voltage differences of 0.05 volt + with no harm done, but i would never recommend any one to do this and do so at your own risk and please be careful when using multi cell any thing and you should at lest have a DMM (multi meter).
If you think really hard you will find you have at lest 3 or 4 + Lion or li-xx type batteries around your house not including your torch batteries and your RC batteries, gee not long ago i counted 15 odd at my place, laptops, camers, phones, mobile phones and all that jazz and most are driven by simple voltage chargers and there is not a house exploding every two seconds in my neighbourhood, the rare few would be the same as torch batteries, major electrical fault, user error or just supper unlucky, if you make and sell 100 000 000 000 items your bound to get a bad few.
Any way i stand by my comment on post #45 answering the question from post #42 series is safer for charging and more convenient in my opinion and from my experience.